Mozilla has announced that the NPAPI plugins for the Firefox browser platform pass to a better life at the end of next year (2016), but maintain support for Adobe Flash, this exception does the collaboration of the company with Adobe to improve flash experience and further improve the stability and safety.
The reason for all this is an easy answer, HTML5, which gives browsers advanced features that were not available then natively, including the ability to render 3D games without using any plugins.
Other browsers such as Chrome or Edge have started to discard certain support plugins. For example, Chrome has rejected nonessential Flash content by default. With the arrival of Firefox on your 64-bit version of Windows you will see how the popular Mozilla browser will not offer support for plugins.